HP 400F Power Supply Troubleshooting Help Needed!

[David D]

I have an HP 400F that has an issue with the power supply that's got me stumped  .

The original problem was that the power supply is supposed to deliver +26V and -26V and the negative rail was supplying -46V instead. The negative rail works the same as the positive rail, except that it gets its regulation from the +26V supply fed into Q7. Q7 was dead (open) so I replaced it with a new 2N3906 (as indicated in the factory manual). I also replaced all electrolytics with new.

The problem now is that with Q7 and the electrolytics replaced, now I'm only getting -1.4V to ground on the negative supply. Even though they checked ok, I tried replacing Q8 and Q9 and I still only have -1.4V on the negative rail.

Any measured voltages I'm getting that are different from the indicated factory voltages are indicated by the red arrows. Can someone help? What am I missing? 


 

[TheDefpom]

I would say to double check you put the caps in the right way around, with the negative rail ones + going to 0V... easy to mix that up.

Seems Q7 is turned on hard, so maybe check R26, R30, CR12

[David D]

Thanks Scott.

I double checked all of the cap polarities and they're correct. I also checked R26, R30, and CR12 and they all check out ok. I also pulled and checked a couple of the tantalums (C24, C25) to make sure they're not shorting and they're fine.

I disconnected the negative rail jumper to separate the power supply from the rest of the meter and still get the same results, so the problem is isolated to the power supply itself. I did notice that the negative rail voltage starts off with around -8V when you first swith on the meter, but the voltage starts to drop after that.

[Technical Ted]

This is a coincidence because I just bought one and repair it a few days ago! Mine also had a -26V PS issue. My C23 cap was shorted causing the voltage to drop to ~-6V with load. I don't have an answer for your issue other than check all the components/connections in the neg supply circuitry. Here are some general things I learned about HP's documentation for this meter.

1- The latest and greatest copy of the manual I could find was on Keysight's site, although it was a pretty rough scan copy.
2- Make sure you go through the last section of the manual and note any changes for your serial number range. Earlier serial numbers may have different circuitry/components. Mine was a later model 950- series with no changes to worry about.
3- I found a drawing error that I assume got missed when they changed the pre-amp from a negative -26V supply to a +26V supply. The drawing shows -26V going to the attenuator relays when in fact it is feed by +26V (confirmed on my unit with ohm meter and physically following traces.
4- What did you replace Q9 with? Did you find a direct replacement somewhere?
5- Note that if needed and the - jumper has been removed that you can load the circuit with a 560 ohm resistor to properly load the circuit directly. This was key for me. I found this by reading the noted manual. I had read earlier version of the manual and never read this in any of them. Only the one from Keysight's site states this: (paragraph 5-45)

Good luck. I still have the unit on my bench if you want me to check a voltage for you or whatever... don't know how much help I can be.

Ted

[Technical Ted]

Oh, I forgot to mention that CR12 and CR13 are physically swapped in position from the layout diagram. Mine were marked on the board. This most likely depends on the version (serial number range) of your unit. I wanted to mention this because if you checked CR12 you may have actually been checking CR13, so I suggest to double check this.

Good luck,
Ted

[factory]

Something I've noted in other HP TE, the small transistors can be various makes & have different lead pinouts, but all with the same HP part number printed on them, double check the transistor leads are in the right order for the circuit.

There seems to be at least three versions of the A2 PCB, I have a 00400-66504 Rev B in mine (617 prefix), very late one has 00400-66514 (0950A change No. 5), late manual has also change of value for R18 (can't see this mentioned in the change sheets).

Another manual on the web shows another 00400-66504 PCB, but with several diodes missing in the PSU area (01795-1, dated Feb 1965 on some pages), the problem with this manual is the front page with the prefix info does not fit the rest of the manual (01795-2, 1966 printed Jan 1968), plus the change sheets from the later manual have been added, i.e. it seems to be a mess of different manuals. 

David

[Tomorokoshi]

Something I've noted in other HP TE, the small transistors can be various makes & have different lead pinouts, but all with the same HP part number printed on them, double check the transistor leads are in the right order for the circuit.

There seems to be at least three versions of the A2 PCB, I have a 00400-66504 Rev B in mine (617 prefix), very late one has 00400-66514 (0950A change No. 5), late manual has also change of value for R18 (can't see this mentioned in the change sheets).

Another manual on the web shows another 00400-66504 PCB, but with several diodes missing in the PSU area (01795-1, dated Feb 1965 on some pages), the problem with this manual is the front page with the prefix info does not fit the rest of the manual (01795-2, 1966 printed Jan 1968), plus the change sheets from the later manual have been added, i.e. it seems to be a mess of different manuals. 

David

The operation appears to be consistent with Q7 being installed incorrectly. A quick LTspice simulation gets similar behavior if the Q7 base and collector are swapped. Trace the leads on the board back to the proper components. Double-check the datasheet: is the pinout of the 2N3906 top-view or bottom-view?

[Technical Ted]

In my unit, the transistors in question are actually marked with EBC for the leads. Don't know about the earlier versions of the A2 board...

Ted

[David D]

Wow, thanks for all of the responses!  I took all of your suggestions and poked around on it again, and I think I finally found the offending component  .

Firstly, my 400F is a later build with the 0950A serial number prefix. I do have the latest manual downloaded from Keysight, but as Ted mentioned, it's a lousy scan and is still missing some correct info. I had noticed that CR12 and CR13 are in the correct positions and correctly labeled on my board, but are swapped relative to the board layout in the manual. The A2 circuit board in mine is p/n 00400-66514 REV A. The A2 board in the manual is 00400-66504. I also noticed the change in value for R18 from 100 ohms to 806 ohms in the latest rev, as well as the addition of CR24 across the base and emitter of Q6 (can someone explain the reason for that to me?).

I verified the transistor pinout as suggested and they match the original components that were removed. The original Motorola transistors had EBC clearly printed on them. The replacements that I obtained from Digikey did not have the pinout marked, but the data sheet indicated that they were the same, as well as a quick check using the diode function on my Fluke 8050A.

Ok, down to the nitty-gritty. When I first turned it on today, I noticed that the negative rail voltage started at ~-19V and was drifting down while bouncing around a bit. I started poking around with the voltmeter and noticed that the voltage feeding the base of Q7 was moving ever so slightly, so I measured across C24 (4.7 mfd tantalum) and it was moving quite a bit. I suspected that C24 might be breaking down under power so I lifted one side and my -26V rail came up from -12V to -25.3V  . I didn't have a 4.7mfd tantalum handy, so I stuck a 4.7 electrolytic in there temporarily and the negative rail has maintained -25V since.

The only thing I'm not sure about is how critical it is that my positive and negative rail voltages match exactly.    I'm reading +26.27 volts on the positive rail and -25.65 volts on the negative rail. Is that any cause for concern?

[David D]

Oh, I forgot to mention to Ted that the Q9 transistor was replaced using a 2N3053 from Digikey. The part number for Q9 was listed in the latest manual parts list.

[Technical Ted]

If you like YouTube videos you might enjoy watching this one on his HP 400F.



Ted

[factory]

In my unit, the transistors in question are actually marked with EBC for the leads. Don't know about the earlier versions of the A2 board...

Ted

Here is the 00400-66504 Rev B board in mine (617 prefix), picture was taken a few years ago, I can't tell you what part the small dome style transistors are ATM.
It has a few signs of older repairs, one of the Sprague 150D tubular solid tants is now a crappy bead tant.

David

[Technical Ted]

This seems like a good time to share, so here's mine. Sorry, I removed the -26V jumper and soldered in leads for shorting and connecting a 560 ohm resistor to load the supply during troubleshooting so you might have trouble reading the part number. My 400F is a 0950- (no A designation) series serial number. I have a scanned copy of what I believe is the correct version of the manual for this unit which is a huge file but much better quality. The newest manual from Keysight's site does have a little more information than this version though... i.e. using the 560 ohm resister to load the power supply... The nerd that I am I actually read both. 

Ted

[David D]

In my unit, the transistors in question are actually marked with EBC for the leads. Don't know about the earlier versions of the A2 board...

Ted

Here is the 00400-66504 Rev B board in mine (617 prefix), picture was taken a few years ago, I can't tell you what part the small dome style transistors are ATM.
It has a few signs of older repairs, one of the Sprague 150D tubular solid tants is now a crappy bead tant.

David

Better a crappy bead tant than a shorted tubular one!    Luckily, the Sprague tubulars are still available on Mouser and Digikey. I just gotta figure out what else to order since I hate spending that much money on shipping for one capacitor... 

[David D]

This seems like a good time to share, so here's mine. Sorry, I removed the -26V jumper and soldered in leads for shorting and connecting a 560 ohm resistor to load the supply during troubleshooting so you might have trouble reading the part number. My 400F is a 0950- (no A designation) series serial number. I have a scanned copy of what I believe is the correct version of the manual for this unit which is a huge file but much better quality. The newest manual from Keysight's site does have a little more information than this version though... i.e. using the 560 ohm resister to load the power supply... The nerd that I am I actually read both. 

Ted

I wish I had the patience to read all the manuals cover to cover, but my attention span is only getting worse the older I get    My doctor says that's normal for someone with ADHD. It might be "normal" in my case, but as my grandfather used to say, "Getting old isn't embarassing, it's just damned inconvenient". 

I didn't load my power supply output after I removed the jumper since it was evident that loading wasn't the issue with mine.

Here's my board mid-troubleshooting:

[David D]

Well, I replaced tantalum C24 and my negative side is back up and running. It wasn't dead shorted, but breaking down under power and pulling the negative rail down.

I do have one question regarding the positive and negative rails: My positive side measures +26.27 volts and my negative side measures -25.65 volts when referenced to ground. Is there any cause for concern that there's a .62 volt difference between them? 

[David D]

Anyone?

I have some meter deflection on all ranges with no input (shorted) and want to rule out the power supply before I delve into checking other components.

[factory]

The manual gives a tolerance of ±1V for both the +26V & -26V supply, your readings are fine, they are never going to be perfect without adjustment pots, which HP deemed not necessary.

David

[David D]

Thanks David!

That table isn't in the version of the manual for my unit (s/n 0950A). I looked in an older version (734-) and it's there. Makes you wonder why HP went backwards on their manual 

Thanks again for the help. I appreciate it!